1. Field of the Invention
The present invention relates to a webbing lock device for a webbing retractor which is used for a seatbelt system for protecting a seat occupant of a vehicle at the time of a sharp deceleration of the vehicle speed and is adapted to prevent the drawing out of the webbing for restricting the occupant at the time of a sharp deceleration of the vehicle speed.
2. Description of the Related Art
In a seatbelt system, one end of a webbing for restricting a seat occupant of a vehicle is taken up by a webbing retractor which incorporates a webbing lock device for preventing the drawing out of the webbing at the time of a sharp deceleration of the vehicle speed.
This lock device is provided with a lock wheel which is adapted to follow the rotation of a takeup shaft in the direction of the drawing out of the webbing by a predetermined urging force. The arrangement is such that, at the time of a sharp deceleration of the vehicle, a pawl lever is engaged with the lock wheel through an acceleration detecting means so as to cause a delay in the rotation of the lock wheel so that lock teeth which rotate together with the takeup shaft are made to project radially from the takeup shaft, thereby causing the lock teeth to mesh with lock teeth of a frame (refer to Japanese Patent Publication No. 16969/1969 and Japanese Patent Laid-Open No. 1115/1972).
With such a conventional lock device, however, if the vehicle is run a bumpy, rough road, there are cases where the acceleration detecting means remains actuated, with the result that the lock device remains in a looked state, thereby making it impossible for the occupant to draw out the webbing. Furthermore, during running on a rough road, the occupant undergoes up-and-down motion repeatedly due to vibrations. As a result, the meshing position between the lock pawl and the lock teeth gradually advances when the webbing is taken up, so that the webbing is gradually taken up by the retractor, possibly increasing the pressure applied to the occupant.
Consequently, the present applicant has proposed a lock device for a webbing retractor which prevents the webbing from being taken up gradually by the retractor even when the webbing is taken up and drawn out repeatedly by a predetermined length (refer to Japanese Utility Model Laid-Open No. 161160/1986).
The lock device in accordance with the aforementioned art comprises a ratchet wheel which rotates by receiving the rotating force of the takeup shaft via a frictional force and has a ratchet wheel formed around its outer periphery which corresponds with the pawl lever, and the pawl lever is forcedly spaced apart from the lock wheel by means of the ratchet wheel when the takeup shaft rotates in the webbing taking-up direction, thereby preventing the webbing from being kept taken up.
However, this arrangement requires a frictional member and associated parts for imparting the frictional force between the takeup shaft and the ratchet wheel, resulting in an increased number of parts used and a decline in efficiency in the assembling operation. In addition, the frictional force of the frictional member must be set in such a manner as to be greater than a predetermined frictional force imparted between the lock wheel and the takeup shaft and capable of positively pressing down the pawl lever to cause the same to be spaced apart from the lock wheel. On the other hand, if this frictional force is made excessively large, wear between the contact surfaces of the takeup shaft and the ratchet wheel is disadvantageously promoted when the takeup shaft and the ratchet wheel rotate relative to each other after the actuation of the pawl lever.